Method of measuring layer thickness and composition of alloy plating

ABSTRACT

Methods of measuring the thickness or thicknesses and the composition or compositions of an alloy plating having one layer or two layers different in composition from each other, wherein the alloy plating includes a metal identical with a substrate metal, suitable for use in analyzing a Zn--Fe group one layer or two layer alloy-plated steel plate. Characteristic X-rays and white X-rays irradiate an object (112, 10, 210) to be measured. A diffraction angle (2θ) of diffracted X-rays of the characteristic X-rays, which are diffracted by an intermetallic compound of the alloy plating, is measured; the composition of the alloy plating is measured from the diffraction angle, an intensity or intensities of fluorescent X-rays from an object to be measured are detected, wherein the flourescent X-rays generated by the white X-rays; and the thickness or thicknesses of the alloy plating are measured from the intensity or intensities of the fluorescent X-rays and the composition of the alloy plating.

TECHNICAL FIELD

This invention relates to a method of measuring a layer thickness andcomposition of an alloy plating, and more particularly to a method ofmeasuring the layer thickness and the composition of an alloy platingincluding a metal identical with a substrate metal. This method issuitable for use in the analysis of a Zn--Fe group alloy-plated steelplate of one or two layers.

BACKGROUND ART

Various plated steel plates having excellent anticorrosion properties,workability, coating properties, weldability and the like have beendeveloped for use in motor vehicle bodies, electrical householdappliances and building materials, and are widely used. In order tostabilize the production quality of these plated steel plates, it isessential to analyze the thickness (deposit value) and the composition(content) of the plating to carry out the process control.

For a steel plate having a plating composed of a content other than Fe,such as a Zn plated steel plate and a Zn--Ni alloy-plated steel plate,the thickness and the composition of the plating can be comparativelyeasily analyzed using fluorescent X-rays, and apparatus for this type ofanalysis has been put to practice.

However, analysis of a Zn-Fe group alloy-plated steel plate, theoutstanding characteristic feature of which has been noted recently, hasbeen impossible by the ordinary method of fluorescent X-rays because theintesity of the Zn fluorescent X-rays is varied due to the content of Znor Fe in the plating and the thickness of the plating. In addition, forFe, a large quantity of Fe fluorescent X-rays are generated from asubstrate steel plate, and these fluorescent X-rays cannot bediscriminated from the Fe fluorescent X-rays in the plating.

For this reason, the following methods of analyzing the Zn--Fe group onelayer alloy-plated steel plate have heretofore been proposed. One methodis proposed in Japanese Patent Laid-Open No. 24680/1980, wherein, usinga Zn plated steel plate which has been subjected to the Zn--Fe alloyingprocess, a fluorescent X-ray intensity of a metal other than Fe, i.e.that of Zn, is measured by two measuring angles differing from eachother. Using predetermined simultaneous equations on the basis of bothmeasured values, the thickness of the plating on the plated steel plateand the degree of alloying (Fe content) are obtained. According to thismethod, Zn fluorescent X-ray intensities of a sufficiently thick pure Znsample are previously measured by two measuring angles, andsubsequently, a Zn fluorescent X-ray intensity of a Zn alloy-platedsteel plate is measured by the same X-ray spectroscope. Analysis is madeof the ratio with the pure Zn fluorescent X-ray intensity obtainedpreviously at the respective measuring angles. The fluorescent X-rayquantitative method wherein the measuring angles are varied has been thebasic theory of the method of analyzing the fluorescent X-rays publishedin textbooks from old times and is well known to everyone.

A second method is proposed in Japanese Patent Laid-Open No.223047/1983, wherein an Fe content in the plating of a Zn--Fealloy-plated steel plate is obtained from an Fe fluorescent X-rayintensity by a first excitation ray incident angle and the fluorescentX-ray measuring angle, both of which Fe fluorescent X-rays from thesubstrate steel plate are not substantially detected. The thickness ofthe plating is obtained from an Fe fluorescent X-ray intensity by asecond excitation ray incident angle and a fluorescent X-ray measuringangle, both of which Fe fluorescent X-rays from the substrate steelplate can be detected.

However, since a plated steel plate flows at a speed as high as 100meter/min, for example on a production line, the plated steel plate isbound to flutter more or less. The influence of this fluttering isreceived only by the steel plate, so that, according to the formermethod proposed in Patent Laid-Open No. 24680/1980, the analyzingaccuracy is necessarily deteriorated.

In general, the thickness of the plating of the alloy-plated steel plateis as thin as 20-30 g/m² (about 3-4 micrometer). If the fluorescentX-rays are strong enough to accurately measure a metal in the plating bythe latter method proposed in Patent Laid-Open No. 233047/1983, then itis impossible to excite only the thin plated layer, and the substratemetal as well as the thin plated layer are bound to be excited. Inconsequence, both the fluorescent X-ray intensities of Fe in the platingand Fe in the substrate steel plate are measured, so that an accurateanalysis cannot be made. Further, when an on-line systemizing isintended, with a low incident angle=measuring angle=5° adopted in thefirst X-ray optical system, construction of such proposed apparatuses asdescribed above have been impracticable, because of the size of an X-raytube, construction of an X-ray spectroscopic system, a protective coverof an analysis meter and the like.

In consequence, both the methods which have been proposed aredisadvantageous. Therefore, a method of chemical analysis has beenrelied on wherein only the plated layer is dissolved and removed byelectrolysis or a suitable acid. The thickness of the plating isobtained from the value of removal, and an Fe content in the plating isobtained through chemical analysis of the value of Fe in the solution.However, it is extremely difficult to dissolve only the plated layerwithout dissolving the substrate steel plate. It requires a considerablyhigh skill level and a long period of time to conduct this chemicalanalysis method. Moreover, this analysis is a destructive analysis tocollect samples from a product and the on-line systemizing cannot beattained, thus presenting such a disadvantage that the reflection ofmeasured results to the process control is delayed to a great extent.

It is very difficult to analyze even the Zn--Fe group of a one layeralloy-plated steel plate as described above. For a Zn--Fe group twolayer alloy-plated steel plate having a first layer with a Zn--Fe alloyplating and a second layer with an Fe plating having Fe as the chiefcontent, the problem becomes further complicated and measurement withhigh accuracy becomes extremely difficult.

In an invention analogous to the present invention, apparatus forcontinuously inspecting a quality of steel plate has been proposed inJapanese Patent Laid-Open No. 17695/1975. The apparatus thereofcomprises: means for causing characteristic X-rays and white X-rays toirradiate a continuously moving steel plate at a predetermined angle;means for detecting diffracted X-rays which have wavelengths satisfyinga condition of Bragg and fluorescent X-rays from this irradiating point;and means for analyzing these detection signals to sense intensityvalues of respective aggregate structures, elements, etc. However, thisinvention is different in object and constitution from the presentinvention, and moreover, the detection of the thickness of plating isnot conducted.

The present invention has been developed to obviate the disadvantages ofthe prior art and has as one object the provision of a method ofmeasuring the thickness and the composition of an alloy plating, whereinthe thickness and the composition of a one layer alloy plating whichincludes a metal identical to a substrate metal can be measuredsimultaneously and non-destructively.

The present invention has as another object the provision of a method ofmeasuring the thickness and the composition of an alloy plating, whereinthe thickness and the composition of an alloy plating having two layersdifferent in composition from each other and which include a metalidentical to a substrate metal, can be measured simultaneously andnon-destructively.

DISCLOSURE OF THE INVENTION

According to the present invention, a fluorescent X-ray analyzing methodand an X-ray diffracting method are simultaneously used to measure thethickness and the composition of a one layer alloy plating whichincludes a metal identical with a substrate metal. As shown in FIG. 1,the characteristic X-rays and the white X-rays irradiate an object to bemeasured; a diffraction angle of diffracted X-rays of the characteristicX-rays, which are diffracted by an intermetallic compound of the alloyplating, is detected; the composition of the alloy plating is measuredfrom the diffraction angle; an intensity of fluorescent X-rays from ametal different from the substrate metal in the alloy plating, whichfluorescent X-rays are generated by the white X-rays, is detected; andthe thickness of the alloy plating is simultaneously measured from theintensity of the fluorescent X-rays and the composition of the alloyplating. With this process, the thickness and the composition of the onelayer alloy plating can be measured simultaneously andnon-destructively.

Furthermore, according to the present invention, the thickness and thecomposition of an alloy plating having two layers different incomposition from each other and including a metal identical with asubstrate metal can be measured. As shown in FIG. 2, characteristicX-rays and white X-rays irradiate an object to be measured; adiffraction angle of diffracted X-rays of the characteristic X-rays,which are diffracted by an intermetallic compound of a first layer(bottom layer) of the alloy plating, is detected; the composition of thefirst layer of the alloy plating is measured from the diffraction angle;intensities of fluorescent X-rays from a substrate metal, a metalidentical with the substrate metal in the alloy plating and a metaldifferent from the substrate metal in the alloy plating, whichfluorescent X-rays are generated by the white X-rays, are respectivelydetected by two measuring angles; and the thickness of plating of thefirst layer and a second layer (top layer) are simultaneously measuredfrom the intensities of the fluorescent X-rays and the composition ofthe first layer of the alloy plating. With this process, in one cycle ofmeasuring, the thickness and the compositions of the alloy platinghaving two layers of different composition and including a metalidentical with the substrate metal are simultaneously measured.

Further, a specific form of the present invention is an arrangementwherein the characteristic X-rays and the white X-rays are generatedfrom a single X-ray source, for example an X-ray tube of Cr target, sothat the measuring apparatus can be simplified in construction.

Another specific form of the present invention is an arrangement whereinthe characteristic X-rays are generated from an X-ray tube of Cr and thewhite X-rays are generated from an X-ray tube of W target, so thatstrong white X-rays can be generated.

A further specific form of the present invention is an arrangmentwherein the incident angles of the X-rays are minimized in values, sothat the intrusion of the X-rays into the substrate metal becomesshallow, thereby increasing information from the alloy plating.

A detailed description of the method of the present invention will begiven by way of example of an Zn--Fe group two layer alloy-plated steelplate, which has a first layer of a Zn--Fe alloy plating and a secondlayer of a Fe plating having Fe as the chief constituent.

In analyzing the above-described two layer alloy-plated steel plate,initially a well known method of varying the measuring angles isconsidered. This method is disadvantageous in that, as described above,with the Zn--Fe alloy-plated steel plate, only the intensities of Znfluorescent X-rays are compared with pure Zn. To obviate thisdisadvantage, a study has been made of a method of using Fe fluorescentX-rays. This is because the fluttering of the alloy-plated steel plateon the production line affects the Fe fluorescent X-rays as well as theZn fluorescent X-rays, whereby, when the analysis is made by use of bothX-rays, the adverse influence can be eliminated.

Now, as shown in FIG. 3, when X-rays are caused to fall into the Zn--Fegroup two layer alloy-plated steel plate 10 at an incident angle of ψ,the whole intensities of fluorescent X-rays of an element to bemeasured, which is measured by a measuring angle ψ₁, can betheoretically represented by the following equation:

    X=X.sub.1 +X.sub.2 +X.sub.3                                ( 1)

where

X₁ is an intensity of fluorescent X-rays of an element to be measuredfrom the second layer (top layer),

X₂ is an intensity of fluorescent X-rays of an element to be measuredfrom the first layer (bottom layer), and

X₃ is an intensity of fluorescent X-rays of an element to be measuredfrom the substrate steel plate.

X₁, X₂ and X₃ are represented by the following equations when thethickness of the second layer (plating deposit value) is T₁, thethickness of the first layer (plating deposit value) is T₂, the contentof the element to be measured in the second layer is W₁, the content ofthe element to be measured in the first layer is W₂ and the content ofthe element to be measured in the substrate steel plate is W₃ : ##EQU1##where ki is a constant, and μ_(i) ^(P) and μ_(i) ^(S) (i=1-3) are massabsorption coefficients of the respective layers to the incident rays(P) and the measured rays (S) respectively.

When a mass absorption coefficient of the element to be measured is(μ/p) and a mass absorption coefficient of the coexistent element is(μ/p)', μ_(i) ^(P) and μ_(i) ^(S) are represented by the followingequations:

    μ.sub.i.sup.P =W.sub.i X(μ/p).sup.P +(100-W.sub.i) X(μ/p).sup.P '(5)

    μ.sub.i.sup.S =W.sub.i X(μ/p).sup.S +(100-W.sub.i) X(μ/p).sup.S '(6)

Since the incident angle ψ is made constant for practical use, the termsof the coefficient (ki/sin ψ) of the right members of theabove-mentioned equations (1)-(3) become constant, too.

In consequence, as shown in FIG. 4, the white X-rays generated from anX-ray tube 12 irradiate a Zn--Fe group two layer alloy-plated steelplate 10 at an incident angle, and, if the intensity of fluorescentX-rays generated is simultaneously measured by detectors 18₁ and 18₂ forZn fluorescent X-rays and detectors 20₁ and 20₂ for Fe fluorescentX-rays through spectroscopic crystals 14₁ and 14₂ for Zn fluorescentX-rays and spectroscopic crystals 16₁ and 16₂ for Fe fluorescent X-rays,which are provided in two measuring angles ψ₁ and ψ₂, then, simultaneousequations on Zn are established by the two measuring angles ψ₁ and ψ₂and also simultaneous equations on Fe are established by the twomeasuring angles ψ₁ and ψ₂. Therefore, a converging thickness Ti and acontent Wi of an element to be measured, the errors of which areminimized, are obtained from the two types of simultaneous equations.However, only two unknown terms can be solved from the aboveinformation. In consequence, according to the above-described method,the analysis can be made only on one layer after another. Then, firstly,after the plating of the first layer, the intensity of the fluorescentX-rays is measured, the thickness T₂ and the content W₂ of the elementto be measured of the first layer plating are obtained from thecombination of the aforesaid theoretical equations. Subsequently, afterthe plating of the second layer, the intensity of the fluorescent X-raysis also measured, and analyzed values of the thickness T₂ and thecontent W₂ of the first layer are inserted to obtain the thickness T₁and the content W₁ of the element to be measured of the second layerplating. However, according to the above-described method, in the caseof on-line systemizing, two analysis devices are required. Moreover,after the plating of the first layer, the measurement is conducted inthe wet state, whereby the intensities of fluorescent X-rays of Zn andFe are varied due to a difference in absorption value of the incidentX-rays and the fluorescent X-rays by the value of moisture on the platedsteel plate, a variation in scattering value of Compton electrons and soon, so that accurate analysis is difficult to make. Naturally, loweredanalyzed values of the first layer lead to deteriorated analyzed valuesof the second layer.

In general, in the case of the Zn--Fe group two layer alloy-plated steelplate which has a first layer of a Zn--Fe alloy plating and a secondlayer of a Fe plating having Fe as the chief constituent, the analysisis needed for three values, i.e. the thickness T₂ and the composition(content) W₂ of the first layer and the thickness T₁ of the secondlayer. In consequence, when one of these is analyzed by some method,other values can be obtained by solving the above-mentioned simultaneousequations.

Then, the inventor of this invention has thought of simultaneously usingthe X-ray diffraction method. More specifically, in respective phasesformed by the Zn--Fe intermetallic compound, the X-ray diffraction angle2θ is varied proportionally to the content of Fe (or Zn) respectively.FIG. 5 shows an example of the variation. In consequence, if therelationship between the X-ray diffraction angle 2θ and the Fe content(%) is as shown in FIG. 5 on an arbitrary lattice plane of a phase(crystal) formed in the Zn--Fe alloy-plated layer (the first layer),then the Fe content (%) in the first layer (or Zn content (%)=100- Fecontent (%) can be obtained by measuring the X-ray diffraction angle 2θfrom the alloy-plating layer.

At this time, even if the thickness T₁ of the second layer is varied,only the intensity of the diffracted X-ray is changed, and thediffraction angle 2θ is not changed. Furthermore, if the second layer isformed of a Fe--Zn alloy plating different in composition from the firstlayer, this fact leads to a difference in the X-ray diffraction angle 2θdue to the difference in crystalline structure. In consequence, from theX-ray diffraction angles 2θ of the both layers, the respectivecompositions (Fe content) of the first and the second layers can beanalyzed.

The present invention has been invented on the basis of the knowledgedescribed above.

BRIEF DDESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow chart outlining the method of measuring the thicknessand the composition of the one layer alloy plating according to thepresent invention;

FIG. 2 is a flow chart outlining the method of measuring the thicknessesand the compositions of the two layer alloy plating according to theinvention;

FIG. 3 is a sectional view showing the state of incidence of the X-raysto the two layer alloy-plated steel plate and the conditions of thegeneration of the fluorescent X-rays for explaining the principle of thepresent invention;

FIG. 4 is a sectional view showing the theoretical construction of theapparatus for measuring the intensity of the fluorescent X-rays forexplaining the principle of the present invention;

FIG. 5 is a chart showing an example of the relationship between theX-ray diffraction angle and the Fe content for explaining the principleof the present invention;

FIG. 6 is a block diagram showing the arrangement of an embodiment ofthe apparatus for measuring the thickness and the composition of thealloy plating of the Zn--Fe one layer alloy-plated steel plate, which isa first embodiment of the present invention;

FIG. 7 is a chart showing the wavelength distribution of the X-raysgenerated from the Cr target X-ray tube as used in this embodiment;

FIG. 8 is a chart showing an example of the relationship between the Znfluorescent X-ray intensity, the Fe content and the plating depositvalue (thickness of plating), for explaining the principle of thepresent invention; and

FIG. 9 is a block diagram showing the arrangement of an embodiment ofthe apparatus for measuring the thickness and the composition of thealloy plating of the Zn--Fe group two layer alloy-plated steel plate,which is a second embodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Detailed description will hereunder be given of the embodiments of thepresent invention with reference to the drawings.

In the first embodiment of the present invention, the present inventionis applied to the measurement of the thickness and the composition ofthe alloy plating of a Zn--Fe one layer alloy-plated steel plate. Theembodiment of the measuring apparatus is constructed as shown in FIG. 6.

In this first embodiment, a Zn--Fe one layer alloy-plated steel plate112 which has been plated in a plating bath 110 is being conveyed in adirection indicated by an arrow A. A powerful X-ray tube 114 forgenerating characteristic X-rays having a suitable wavelength, such asCr target, is provided at a proper position on a horizontal conveyingsection (or a vertical conveying section) for the Zn--Fe one layeralloy-plated steel plate 112. X-rays emitted from the X-ray tube 114 aremade to fall onto the Zn--Fe one layer alloy-plated steel plate 112 atan incident angle through a solar slit 116. Then, the X-rays arediffracted through the following formula of Bragg by respective crystallattice planes of the phases of the Zn--Fe intermetallic compound formedin the alloy plating layer of the Zn--Fe alloy-plated steel plate 112:

    λ=2d sin θ                                    (7)

where λ is a wavelength, θ is an angle, and d is the distance betweenthe crystal lattice planes of the Zn--Fe intermetallic compound.

At this time, in the crystal of the Zn--Fe intermetallic compound, thelattice constant varies due to the content of Fe (or Zn), whereby theX-ray diffraction angle 2θ is deviated. More specifically, since theX-ray diffraction angle 2θ is varied due to the content of the Fe (orZn), the relationship between the content (%) of Fe (or Zn) and theX-ray diffraction angle 2θ is obtained on an arbitrary crystal latticeplane, as shown in an example in FIG. 5, by a solar slit 117, agoniometer 118, a diffracted X-ray detector 120 and a counter circuit122. When the X-ray diffraction angle 2θ is measured by the same crystallattice plane from the layer of the alloy plating, the Fe content (%) inthe alloy plating (or the Zn content (%)=100-Fe content (%)) in thealloy plating of the Zn--Fe one layer alloy-plated steel plate 112 canbe obtained.

On the other hand, since the X-ray tube 114 generating thecharacteristic X-ray normally generates the white X-rays as well, asshown in FIG. 7, the white X-rays are effectively utilized in this firstembodiment. More specifically, the white X-rays excite a metal containedin the Zn--Fe one layer alloy-plated steel plate 112, and generate thefluorescent X-rays having a wavelength commensurate to the metal, sothat the analysis is then made by use of the Zn fluorescent X-rays. Morespecifically, from the fluorescent X-rays of the metal contained inZn--Fe one layer alloy-plated steel plate 112, which metal is excited bythe white X-rays, only the Zn fluorescent X-rays are spectroscopicallydivided by a spectroscopic crystal 126 through a solar slit 124, and theintensity of the Zn fluorescent X-rays is measured by a fluorescentX-ray detector 130 and a counter circuit 132 through a solar slit 128.

Since the intensity of the Zn fluorescent X-rays, which is detected bythis fluorescent X-ray detector 130, is varied due to the Zn (or Fe)content and the thickness of the Zn--Fe one layer alloy plating asdescribed above, the analysis cannot be made by a normal method.However, as described above, the Zn content in the alloy plating can beanalyzed by the X-ray diffraction method, so that the thickness ofplating can be obtained from the Zn fluorescent X-ray intensity.

FIG. 8 shows an example of a study of the relationship between the Znfluorescent X-ray intensity and the thickness of plating (platingdeposit value). It is apparent that, if the Fe content (%) is found,then the thickness of plating can be easily known from the Znfluorescent X-ray intensity. At this time, the thickness of plating maybe obtained in such a manner that an influence quantity of Fe to the Znfluorescent X-ray intensity is previously obtained and is corrected bythe Fe content.

Additionally, it would be effective if the incident angle ψ of theX-rays would be minimized in consideration of the influence offluttering and the X-ray diffraction angle 2θ of the measuring latticeplane, because the intrusion of the X-rays into the substrate steelplate becomes shallow, and consequently, information from the alloyedplating is increased.

A series of X-rays diffraction intensities obtained by scanning todetect the diffraction angle 2θ of an arbitrary crystal lattice planeaccording to the above-described X-ray diffraction method and theaforesaid fluorescent X-ray intensity are simultaneously measured andinputted to a computer 134, respectively. In this computer 134, variouscalculations are performed and the Fe content and the thickness ofplating in the alloy plating are obtained. The results are displayed inan indicator 136 on the line of the spot and inputted to a processcomputer 138 simultaneously, whereby plating conditions of the platingbath 110 and the like are controlled on the basis of the analyzedvalues.

In this first embodiment, both the characteristic X-rays and the whiteX-rays are generated from the single X-ray tube 114, so that themeasuring apparatus is simplified in construction. Additionally, themethod of generating the characteristic X-rays and the white X-rays neednot necessarily be limited to this, and two X-ray tubes including anX-ray tube of Cr target, for example, for generating the characteristicX-rays, and an X-ray tube of W target, for example, for generating whiteX-rays high in intensity, may be used or other X-ray sources may beutilized.

In the first embodiment, the present invention has been applied to themeasurement of the thickness and the composition of alloy plating of theZn--Fe one layer alloy-plated steel plate; however, the scope ofapplication of the present invention need not necessarily be limited tothis, and it is apparent that the present invention may be applied toall of the measurements of the thickness and the composition of onelayer alloy plating including a metal identical with the substratemetal.

Detailed description will hereunder be given of one embodiment of theapparatus for measuring the thickness and the composition of plating ofthe Zn--Fe group two layer alloy-plated steel plate, which is the secondembodiment of the present invention.

This second embodiment is constructed as shown in FIG. 9, wherein aZn--Fe two layer alloy-plated steel plate 210, which has a Zn--Fe alloyplating applied thereto in a first plating bath 222 and a Fe platinghaving Fe as a main constituent applied thereto in a second plating bath224, is being conveyed in a direction indicated by an arrow A. Apowerful X-ray tube 212 for generating characteristic X-rays having asuitable wavelength, such as Cr target, is provided at a proper positionon a horizontal conveying section (or a vertical conveying section) forthe Zn--Fe group two layer alloy-plated steel plate 210, and X-raysemitted from the X-ray tube 212 are made to fall onto the Zn--Fe grouptwo layer alloy-plated steel plate 20 at an incident angle ψ. Then, theX-rays are diffracted through the formula of Bragg shown in (7) abovedescribed by respective lattice planes of the phases (crystal) of theZn--Fe intermetallic compound formed in the first layer of the Zn--Fegroup two layer alloy-plated steel plate 210.

In consequence, when the X-ray diffraction angle 2θ is measured by anX-ray diffraction device 226 including a goniometer 226A, thecomposition of the first layer, i.e. the Zn--Fe alloy-plated layer, canbe analyzed from the relationship shown in FIG. 5.

On the other hand, the white X-rays are also generated from the X-raytube 212 as shown in FIG. 7, whereby, the white X-rays excite Zn and Fein the Zn--Fe group two layer alloy-plated steel plate 210 to generatefluorescent X-rays. The intensity of the generated fluorescent X-rays issimultaneously measured by use of spectroscopes having two measuringangles ψ₁ and ψ₂, which are different from each other for both Zn andFe, in a fluorescent X-ray analysis device 228 having the basicconstruction as shown in FIG. 4.

Also in this second embodiment, it would be effective if the incidentangle ψ of the X-rays would be minimized in consideration of theinfluence of the fluttering and the X-ray diffraction angle 2θ of themeasuring lattice plane, because the intrusion of the X-rays into thesubstrate steel plate becomes shallow, and consequently, informationfrom the alloy plating is increased.

Values measured by the X-ray diffraction device 226 and the fluorescentX-ray analysis device 228 are inputted to a computer 230, where thecomposition of the first layer is analyzed firstly, the above-describedsimultaneous equations are solved by use of the analyzed values, and thethicknesses T₁ and T₂ of the first and the second layers are obtained.The results are displayed in an indicator 232 on the line of the spotand inputted to a process computer 234 simultaneously, and platingconditions of the plating baths 222 and 224 and the like are controlledon the basis of the analyzed values.

In this second embodiment also, the characteristic X-rays and the whiteX-rays are generated from the single X-ray tube 212, so that themeasuring apparatus is simplified in construction. Additionally, themethod of generating the characteristic X-rays and the white X-rays neednot necessarily be limited to this, and two X-ray tubes including anX-ray tube of Cr target, for example, for generating the characteristicX-rays, and an X-ray tube of W target, for example, for generating whiteX-rays high in intensity, may be used or other X-ray sources may beutilized.

In the second embodiment as described above, the present invention hasbeen applied to the measurement of the thickness and the compositions ofalloy plating of the Zn--Fe group two layer alloy-plated steel plate;however, the scope of application of the present invention need notnecessarily be limited to this, and it is apparent that the presentinvention may be applied to the measurement of all of the thicknessesand the compositions of plating having two layers different incomposition from each other and including a metal identical with thesubstrate metal.

INDUSTRIAL APPLICABILITY

As has been described hereinabove, according to the present invention,it becomes possible to simultaneously and non-destructively measure thethickness and the composition of the one layer alloy plating including ametal identical with the substrate metal, of the Zn--Fe alloy-platedsteel metal or the like, which has heretofore been very difficult tomeasure. In consequence, it becomes possible to measure on-line thethickness and the composition of the one layer alloy plating andimmediately feed back analyzed results to the line, thus providing anoutstanding contribution to the stable operation and improved quality ofthe Zn--Fe one layer alloy-plated steel plate and the like.

Furthermore, according to the present invention, it becomes possible tosimultaneously and non-destructively measure the thicknesses and thecompositions of respective layers of the alloy plating having two layersdifferent in composition from each other and including a metal identicalwith the substrate metal, of the Zn--Fe group two layer alloy-platedsteel plate, which has heretofore been very difficult to measure. Inconsequence, it becomes possible to conduct the analysis of thethicknesses and the compositions of the respective layers by a singleapparatus and achieve the on-line systemizing. Hence, the analyzedresults can be immediately fed back to the line and an outstandingcontribution can be made to the stable operation and improved quality ofthe Zn--Fe group two layer alloy-plated steel plate and the like.

I claim:
 1. A method of measuring the thickness and the composition of aone layer alloy plating including a metal identical with a substratemetal, said method comprising the steps of:irradiating an object to bemeasured with characteristic X-rays and white X-rays; detecting adiffraction angle of diffracted X-rays of said characteristic X-rayswhich are diffracted by an intermetallic compound of said alloy plating;measuring the amount of the composition of said alloy plating from saiddiffraction angle; detecting an intensity of fluorescent X-rays from ametal different from said substrate metal in said alloy plating, thefluorescent X-rays being generated by the white X-rays; and measuringthe thickness of said alloy plating from the intensity of thefluorescent X-rays and the amount of the composition of said alloyplating.
 2. A method of measuring the thickness and the composition ofalloy plating as set forth in claim 1, wherein said one layer alloyplating is a Zn--Fe one layer alloy plating applied onto a steel plate.3. A method of measuring the thickness and the composition of alloyplating as set forth in claim 1, wherein said characteristic X-rays andsaid white X-rays are generated from a single X-ray source.
 4. A methodof measuring the thickness and the composition of alloy plating as setforth in claim 3, wherein said single X-ray source is an X-ray tube witha Cr target.
 5. A method of measuring the thickness and the compositionof alloy plating as set forth in claim 1, wherein said characteristicX-rays are generated from an X-ray tube with a Cr target and said whiteX-rays are generated from an X-ray tube with a W target.
 6. A method ofmeasuring the thickness and the composition of alloy plating as setforth in claim 1, wherein an incident angle of said characteristic andsaid white X-rays is minimized.
 7. A method of measuring the thicknessand the composition of an alloy plating having two layers different incomposition from each other and including a metal identical with asubstrate metal, said method comprising the steps of:irradiating anobject to be measured with characteristic X-rays and white X-rays;detecting a diffraction angle of diffracted X-rays of the characteristicX-rays which are diffracted by an intermetallic compound of a firstlayer of the alloy plating, said first layer being adjacent to thesubstrate of said object; measuring the amount of the composition ofsaid first layer of the alloy plating from said diffraction angle;detecting intensities of fluorescent X-rays from a substrate metal, ametal identical with said substrate metal in the alloy plating and ametal different from said substrate metal in the alloy plating, saidfluorescent X-rays being generated by said white X-rays, and saidintensities being detected by two measuring angles; and measuring thethickness of said first layer and a second layer of the alloy platingfrom said intensities of said fluorescent X-rays and the amount of thecomposition of said first layer of the alloy plating.
 8. A method ofmeasuring the thickness and the composition of alloy plating as setforth in claim 7, wherein said two layers of alloy plating include afirst layer of Zn--Fe alloy plating and a second layer of Fe platinghaving Fe as the chief content applied onto a steel plate.
 9. A methodof measuring the thickness and the composition of alloy plating as setforth in claim 7, wherein said characteristic X-rays and said whiteX-rays are generated from a single X-ray source.
 10. A method ofmeasuring the thickness and the composition of alloy plating as setforth in claim 9, wherein said single X-ray source is an X-ray tube witha Cr target.
 11. A method of measuring the thickness and the compositionof alloy plating as set forth in claim 7, wherein said characteristicX-rays are generated from an X-ray tube with a Cr target, and said whiteX-rays are generated from an X-ray tube with a W target.
 12. A method ofmeasuring the thickness and the composition of alloy plating as setforth in claim 7, wherein an incident angle of said characteristic andsaid white X-rays is minimized.